Iodine is a non-metallic element of the halogen family and is the only halogen that is solid at ordinary temperatures. Iodine has been shown to have a range in valance of from -1 to +7 and compounds thermodynamically stable with respect to their constituent elements are known to exist for all of the oxidation states of iodine.
Iodine was discovered early in the 19th century and the first practical therapeutic application of iodine was as a remedy for goiter. This use was followed shortly thereafter with the use as a germicide for the treatment of wounds. It was during the American war between the states that the first wide-spread use of iodine as an antiseptic and germicide was developed for the treatment of battle wounds. Since that time, iodine has been recognized to be a preferred germicide but because of certain inherent chemical, physical and biological properties, its antiseptic degerming use for humans and animals has been limited.
Elemental iodine has a high vapor pressure which results in pharmaceutical compositions having variable germicidal potency as the iodine content volatilizes from an antiseptic preparation on aging. Moreover the high vapor pressure of iodine contraindicates its use in closed compartments, such as body cavities or under a bandage because of corrosive destruction of skin, mucous membranes and other vital tissues. While the general systemic toxicity of iodine is low, fatalities have occurred after the ingestion of iodine solutions. However, the pathologic changes recorded for fatal cases of iodine poisoning are largely the result of tissue hypoxia and local corrosive destructive effects rather than systemic iodine poisoning.
Another limitation for the germicidal use of iodine is its high aqueous insolubility (0.034% at 25.degree. C.). While the aqueous solubility of iodine may be increased through the use of alcohol (as for example, tincture of iodine) or through the use of inorganic metallic salts as solubilizing agents (as for example, sodium iodide and/or potassium iodide in the preparation of Lugols' Solution), such iodine solutions also possess the same toxic tissue manifestations which generally limit the use of iodine germicidal solutions.
When alcohol is used as a solvent for iodine, the use of such preparations on abraded and injured skin or mucous membranes is painful and damaging. Further, as the alcohol evaporates, the iodine content concentrates which increases the incidence of burning, corrosive destruction and staining of tissues.
Metallic iodides have been used to solubilize elemental iodine in water through the direct formation of a water-soluble iodine complex formed between the diatomic iodine (I.sub.2) and the iodide ion (I.sup.-) to form I.sub.3.sup.- ions. Such aqueous iodine solutions have not modified the toxic tissue reactions of elemental iodine and burning and staining still occur. In fact, such untoward responses are now more frequent since larger concentrations of elemental iodine are utilized to prepare the aqueous iodine germicidal preparations.
Iodine in aqueous solution dissociates to equilibrate as follows: ##STR1## with the equilibrium constant (K.sub.1) being about 4.times.10.sup.-46 depending on the temperature. In aqueous media, the dissociation phenomena for diatomic iodine is further complicated by the formation of several species of iodide ion, the most significant of which is the tri-iodide ion. The equilibrium constant (K.sub.2) being approximately 7.5.times.10.sup.2. ##STR2##
It is preferred to combine these equilibrium reactions when describing the dissociation of diatomic iodine in aqueous solutions as: ##STR3## with the equilibrium constant (K.sub.3) being approximately 3.times.10.sup.-43.
Iodine is a mild oxidizing agent in acid solution with a redox equilibrium potential of 0.534 V at 25.degree. C. for the iodine-iodide ion couple. Iodine will readily oxidize sulfite to sulfate and thiosulfate to tetrathionate, while ferric and cupric salts are reduced in acid solution by the iodide ion to form free iodine. In dilute solutions, iodine completely oxidizes sulfur dioxide to sulfuric acid, whereas iodides reduce sulfuric acid to sulfur dioxide, sulfur and even hydrogen sulfide, with the liberation of free iodine.
In neutral or slightly alkaline aqueous solutions, iodine exerts a somewhat stronger oxidizing action because of the formation of hypo-iodite ion in accord with the following reaction: EQU I.sub.2 +2OH.sup.- .fwdarw.I.sup.- +IO.sup.- +H.sub.2 O
Such aqueous solutions are strong iodinating agents and cause redox changes in body proteins and other biologic substances within the alkaline physiologic pH range. Iodine will add to unsaturated linkages in tissue proteins to cause denaturation which interrupt essential physiologic reactions.
In an effort to overcome the noxious tissue toxicity observed for aqueous and hydroalcoholic solutions of iodine, while at the same time maintaining the germicidal activity of elemental iodine, water soluble organic complexes of iodine with organic polymers were prepared. The combination of elemental iodine and certain organic polymers, as for example, polyvinylpyrrolidone and detergent polymers, was shown to increase the aqueous solubility of elemental iodine and such polymer-iodine products were termed, iodophors.
The organic polymers used to form an iodophor comprise a broad range in molecular weight and chain length and may be either ionic or non-ionic in character as well as to possess either surfactant or non-surfactant properties. A loose bond forms between the iodine and organic polymer to form the complex and aqueous solutions of up to 30 percent by weight in iodine content may be prepared.
The general class of organic iodophor compounds comprise two distinct polymer groups; the first group consisting of only one member, polyvinylpyrrolidone, which is a non-detergent, non-ionic and non-surface active polymer, the second group comprises the broad variety of detergent-surface active polymers including non-ionic, anionic and cationic surface active polymers. Both polymer groups are complexed with elemental iodine to form the iodophor.
The general method for the preparation of an iodophor complex is to bring into intimate contact, elemental diatomic iodine with the selected polymer either in the dry or powder form or in the presence of a suitable solvent. Heat may be used to accelerate complex formation. Upon completion of the reaction, the iodophor complex of the respective polymeric carrier with iodine is obtained in certain reproducible proportions of one to the other.
The widespread use of iodophor germicidal preparations has now established that problem of iodine tissue irritation; tissue staining and destructive corrosivity have been essentially eliminated through iodophor complexing although the germicidal potency known for elemental iodine remained essentially unchanged.
Studies have demonstrated that the microbicidal potency of iodophor germicidal preparations is essentially the same as that known for aqueous and/or alcoholic solutions of elemental iodine despite the modified tissue toxicity of iodophors. The superiority of the iodophor germicidal preparations over the aqueous and/or alcoholic inorganic elemental iodine solutions was shown to reside essentially in decreased toxicity, reduced tissue irritation, a lowered iodine vapor pressure as well as in the non-staining of skin and natural fabrics.
As the advantage of a lower incidence of untoward noxious tissue responses was established for iodophor germicides, efforts were directed toward developing methods to measure the quantitative extent of complexing between the iodine and the organic polymer in order to formulate iodophor germicidal preparations with improved stability and reproducibility.
Iodophor preparations are described in terms of available or titratable iodine which is considered to be the iodine released from the complex to exert its germicidal action. However, such available iodine determinations do not either reflect the total iodine content of the iodophor or its germicidal potency.
Iodophor solutions are also categorized on the basis of the amount of iodine extracted into an immiscible organic solvent which is expressed as the distribution coefficient for the preparation. Such extracted iodine is defined as uncomplexed or free iodine and is interpreted to reflect iodophor integrity. The uncomplexed iodine is postulated to be the cause of toxic responses, unstable and malodorous preparations and the distribution coefficient became the general measure to predict stability of the preparation on storage as well as the occurrence of toxic tissue responses.
The determination of the distribution coefficient for an iodophor solution, in accord with the teachings of the prior art, involves extracting exactly 1 ml. of an iodophor solution, having previously determined the amount of available iodine, with 25 ml. of heptane in a stoppered glass container. The iodine content in the heptane layer is measured spectrophotometrically at 525 mu. and the distribution coefficient (DC) calculated from the formula: ##EQU1##
The relationship between the distribution coefficient and the quantitative degree of complexing present in an iodophor germicidal preparation was made on an empirical basis from the detection of an iodine odor in certain iodophor solutions but not in others. Iodophor solutions having a strong iodine odor were found to have a distribution coefficient of about 100 when determined by the above method. The iodine odor of such iodophor solutions lessened as the distribution coefficient reflected unsatisfactory polymer-iodine complexing and thereby gave rise to the odor because such solutions were presumed to have a large amount of free uncomplexed iodine. However, when the distribution coefficient for an iodophor solution was determined to be greater than 200, no iodine odor was detected and therefore satisfactory iodine-polymer complexing was assumed to be present. It was taught that the quantitative level of iodine complexing present in an iodophor compound was directly related to the distribution coefficient but inversely related to the strength of iodine odor detected (see for example U.S. Pat. No. 3,028,300).
However, it has been found that this test for iodophor integrity, noxious responses and germicidal potency by imputing the distribution coefficient as a measure of the degree of iodine bonding or polymer-iodine complex formation is in error because it does not recognize the complicated dynamic iodine equilibria present in aqueous iodophor solutions which form multiple thermodynamically stable or metastable states of iodine that are independent of the degree of polymer complex formation.
The dynamic systems present in aqueous iodophor solutions constantly establish new equilibrium states as iodine is removed by immiscible solvent in the course of the test method. The failure of the distribution coefficient to be a measure of iodophor complexing is readily demonstrated by a comparison of the properties of iodophor solutions with different distribution coefficients utilizing the criteria set forth in the art to establish the presence of satisfactory bonding.
Iodophor solutions were prepared in accord with the monograph for povidone-iodine solution of the United States Parmacopeia, Twentieth Revision, and were found to have a wide range in distribution coefficient when determined by the method of the prior art. These solutions were studied for the relationship between the distribution coefficient, intensity of iodine odor and the occurrence of tissue irritation. As a further basis for comparison and control, the solutions were compared with the properties of Strong Aqueous Iodine Solution, U.S.P., also known as Lugols' Solution, which is an uncomplexed inorganic aqueous solution of iodine.
The presence or absence of an iodine odor was determined organoleptically and the degree of iodine odor when present was graded on a scale of from 1+ to 4+ using the uncomplexed Strong Aqueous Iodine Solution as a reference control standard.
The distribution coefficient was determined in accord with the prior art method described above by extracting 1 ml. of the previously titrated U.S.P. povidone-iodine solution with 25 mls. of water-saturated heptane in a stoppered glass container which is maintained at 25.degree. C..+-.I.degree.C. The heptane layer is sampled and the iodine content in this layer determined at 525 mu with a Beckman Model DU-7 Spectro-photometer. The iodine remaining in the aqueous sample layer was calculated by difference. The distribution coefficient (DC) was calculated from the equation: ##EQU2## The Strong Aqueous Iodine Solution serving as a control preparation was treated in identical manner as described above. The test results for the solutions studied are presented in Table I.
The respective test solutions were then studied in rabbits for topical skin irritation. Each of the test solutions was applied to the skin of the shaved back of a rabbit and covered with a gauze patch. The irritation potential of the test solution to rabbit skin was read after 24 hours of contact. The results are reported in Table I.
TABLE I ______________________________________ USP Povidone- Iodine Distribution Rabbit Skin Test Solutions Coefficient Odor Irritation ______________________________________ No. 1 123 No odor No irritation No. 2 44 ++odor No irritation No. 3 35 +++odor Irritating No. 4 338 No odor No irritation No. 5 23 No odor No irritation CONTROL PREPARATION Strong 18 ++++odor Irritating Aqueous Iodine Solution, USP ______________________________________
A comparison of the test results obtained for the respective solutions readily demonstrate that the distribution coefficient does not correlate with the criteria claimed in the prior art. The control solution, Strong Aqueous Iodine Solution, USP, which is an uncomplexed, inorganic aqueous solution, without an organic polymer, has a distribution coefficient of 18; a 4+ iodine odor and caused marked irritation to rabbit skin. Povidone-Iodine test solution No. 4 which had a distribution coefficient of 338 was without iodine odor and without skin irritation. The properties for these solutions are consistent with prior art criteria.
However, when the distribution coefficient determined for other iodophor preparations in the test series were compared with the prior art criteria, we find that the predictive value attributed to the distribution coefficient by the prior art to be inconsistent, variable and essentially meaningless to practice.
Povidone-Iodine test solution No. 5 which has about the same distribution coefficient as the uncomplexed control inorganic iodine solution, the Strong Aqueous Iodine Solution, (i.e., 23 vs. 18) has no iodine odor and does not cause skin irritation. Povidone-Iodine test solution No. 2 has a distribution coefficient of 44 but a 2+ iodine odor also does not cause skin irritation.
Contrasting properties are presented by povidone-iodine solution No. 3 which has a distribution coefficient to 35 and a 3+ iodine odor and is irritating. Povidone-Iodine Solution No. 1 which has a distribution coefficient of 123 which is markedly below 150, is without odor and is non-irritating.
These contrasting and unrelated differences in the response of iodophor solutions, when correlated with the distribution coefficient, demonstrate not only the unreliable meaning imputed to the distribution coefficient as a measure of iodophor integrity but also point to the inherent variability observed for the iodine distribution coefficient determined for iodophor solutions. Such variations in the iodine distribution coefficient for iodophor solutions are well known in the art. This variability in the distribution coefficient for an iodophor solution arise because the same molecular species is not present in both liquid phases.
The distribution coefficient for a solute is defined as the ratio in the amount of solute dissolved in two immiscible liquids at equilibrium. Since the distribution law (Nernst Law) is intended to express the behavior of only a single chemical species as it distributes itself between the two immiscible liquid phases, any tendency for the solute to be abnormally distributed in either phase results in a divergence from the normal distribution coefficient.
The iodine moiety of povidone-iodine complex is present in an aqueous iodophor solution in the form of different thermodynamically stable anionic iodine species and diatomic iodine. The anionic iodone forms are capable of generating diatomic iodine in the course of their respective equilibrium reactions. The anionic species do not distribute themselves into the extracting solvent which removes only the nonionic iodine. Such iodine is generated in the course of the iodine equilibrium reaction and its extraction by the solvent fractionates the equilibrium state. The disturbed equilibrium reaction is soon re-established to restore new anionic iodine species but now at a different concentration level since the previous aqueous iodine content of the solution has been reduced by the extracting solvent.
It is for this reason that the distribution coefficient for iodophor solutions will vary appreciably in practice because these values are determined upon the erroneous presumption that a single molecular species of the solute is present in each phase. The distribution coefficient which is intended to reflect the total concentration of the single solute in each phase is therefore meaningless as a measure of polymer complexing, iodophor integrity and as a predictive means for the occurrence of noxious skin irritations.
Since the iodophor iodine exerting the microbicidal action exists in solution in dynamic equilibrium with ionic iodine species, the removal of one or more of the iodine species results in the formation of new equilibrium forms. The extracting solvent removes or consumes iodine from the iodophor solution in a manner similar to that of the microbial and organic load during the degerming use of the iodophor solution. The amount of iodine available for germicidal action in an iodophor preparation therefore, is the amount of iodine in the equilibrium in the solution at the time of its use. Such equilibrium iodine content represents the germicidal potency of the preparation but not the total iodine content titrated for the preparation nor the apparent distribution of iodine determined for two solvents as taught in the art. Although iodophor solutions are assayed in the art for available or titratable iodine, it is the equilibrium iodine which is the particular form of iodine present in the iodophor solution that is instantly available to exert the microbicidal action. This form of iodine differs from titratable iodine and the other iodine species present in the iodophor solution and the equilibrium iodine content of an iodophor solution is to be distinguished from its titratable iodine content.
The titratable iodine content of an iodophor preparation includes the iodine reservoir of the iodophor preparation povidine iodine to the particular dynamic equilibrium reactions occurring in an iodophor solution as well as the equilibrium iodine in the solution.
Titratable Iodine=Reservoir Iodine+Equilibrium Iodine However, it is the equilibrium iodine alone that exerts the microbicidal action of the preparation at any given moment. The portion of the titratable iodine content remaining after subtracting the amount of equilibrium iodine present serves as the iodine reservoir to generate new equilibrium iodine in solution as it is consumed by the microbial and bio-organic load in the course of microbicidal activity but does not exert such germicidal action by itself.
It is thus necessary to devise a simple method for determining the content of equilibrium iodine in an iodine solution, and thus to have a measure of the germicidal activity of such solution.